Manufacturers use electrical discharge machining (EDM) to remove portions of metal too hard to machine with conventional milling techniques, or to form intricate cavities that could not be easily machined using conventional milling processes in a cost effective manner. During a series of consecutive sparks that produce a series of micro-craters on an electrically conductive work piece in the presence of an energetic dielectric fluid, the EDM process vaporizes material along a cutting path. Wire-cut EDM and die-sinking EDM are the two common types utilized by manufacturers today. In wire-cut EDM, a thin single strand metal wire is fed through a work piece that is constantly fed from a spool and held between an upper and lower guide.
In die-sinking EDM, a graphite or copper electrode is machined into a desired shape and fed into a work piece to erode a cavity in a dielectric fluid. The eroded cavity is bigger than the electrode because of spark gap and EDM orbit. Spark gap is caused by the EDM erosion process itself and is proportional to the amount of current used. The EDM orbit moves the electrode in a programmed path which creates room for the dielectric fluid and eroded material to escape.
The shape and size of the orbital path is based primarily on the size and shape of the electrode. The most commonly used orbits are spherical, circular, square, star and custom. The shape of the cavity being machined is dependent on the shape and size of the orbit. For example, if an electrode with a square cross section is moved in a circular orbit, the resulting cavity would have rounded corners along the vertical edges. Because the size and shape of the resulting cavity should meet particular dimensions, the electrode is often under-sized based on the chosen orbit.
To virtually mill a part, a CAD application, for example NX(tm) by UGS Corp., is utilized to define the electrode path of orbit. To design the actual electrode, however, there are commercially available electrode design packages, for example PS-Electrode by Delcam and Quick Electrode by Cimatron, that completely ignore the steps required for orbit and gap compensation. When designing the electrodes, features in cavities are identified, a negative core is modeled, tangential extensions are formed, and then electrode base and holders are selected. The most important and challenging step in the electrode design process is compensating the geometry for spark gaps and EDM orbits, which said commercially available packages fail to provide. The user is therefore left to use manual offsets and manipulate machining tools to under-size the electrode and trick the milling machine to form an electrode that is smaller than the electrode originally designed to compensate for the inherent deficiencies present today. For example, in circular and spherical orbits, the electrode making process is tricked by under-sizing programmed tools and over-sizing actual milling tools. Circular and spherical orbits are limited since they cannot produce sharp corners in the cavity.
Spherical orbit under-sizing is the easiest to manage, as the spark gap and orbit gap are uniformly applied to the entire electrode geometry. This uniform application is accomplished by using either face offsets or applying negative stock at the machine tool. (during NC programming, negative stock is applied by programming with a smaller tool.) There are severe limitations to both the methods: (1) face offsets are not always reliable, (2) negative offsets can only be applied with tools with corner radii bigger than the offset value, (3) the final cavity will have corner radii as big as 50% of orbit_distance+finish_spark_gap, and (4) smaller corner radii in the original cavity could result in sharp corners on the under-sized electrode that could increase electrode wear and (5) Spherical orbits take at least twice as long as other orbits to erode the desired cavity.
A second category of orbit path is the circular orbit that is technically more complicated to achieve than under-sizing for spherical orbits. In the circular orbit, the orbit gap is applied uniformly in a X-Y plane. There is no orbit gap along a Z axis. Rather, the spark gap is applied uniformly in all directions. If the electrode geometry is simple, most users manage the electrode geometry by offsetting select faces from the electrode. The management of electrode geometry is also accomplished by manipulating the programmed tool while machining the electrode. The limitations are: (1) face offsets are less reliable than in spherical orbit under-sizing, (2) face offsets have to be individually calculated and administered for inclined faces, which makes it even less reliable, (3) the final cavity will have vertical corner radii as big as 50% of orbit_distance+finish_spark_gap, (4) small vertical corner radii in the original cavity could result in sharp corners on the under-sized electrode that could increase electrode wear, and (5) manipulating tool diameter while machining the electrode is only applicable to flat end mills.
The third category of orbit path is the square orbit, which is the most commonly used polygonal planar orbit in Sink EDM. Square orbits produce superior results because they are capable of creating corner radii as small as the finish spark gap in the final cavity. Square orbits also do not cause the sharp corners in the electrode thereby increase electrode longevity. However, designing an electrode for a square orbit is very complicated except in very simple cases. And hence, it is very seldom employed.
FIG. 1 is an orthogonal orientation depicting a prior art technique to undersize an electrode to account for orbital path and spark gap, under-sizing the programmed tools is done manually to account for spark gap and orbit size. For example, a user desires to mill an electrode 100 and uses a cylindrical tool 105, to that end. The tool is 40 millimeters long and 10 millimeters in diameter, for example. Given a spark gap of 0.02 millimeters, and an orbit size of 0.1 millimeter, the user “cheats” the CAM system by programming the tool to be 10 mm−2(spark gap+orbit size), or 9.76 mm. Likewise the length is shortened by just the spark gap to result in 39.98 mm (40 mm−0.02 mm) programmed tool length. Therefore, the milling machine thinks it is using a programmed tool 110, but it is really using the cylindrical tool 105.
There is a need for a solution that can easily undersize electrodes for polygonal orbits to produce sharp corners and without needed manual modification to trick the tools in manufacturing the electrodes.